Dr Joanne Whittaker and Professor
Dietmar Müller's research provides
a framework for understanding the
source of seafloor roughness.
Image: University of Sydney

Dr Joanne Whittaker and Professor Dietmar Müller, from the School of Geosciences, and colleagues in France and USA, have found a previously unknown connection between the break-up of the ancient supercontinent Pangaea and the topography of the deep ocean floor.

The research, published in the journal Nature, reveals for the first time how smooth flat expanses and rough hilly areas of ocean floor form.

Ocean floors have remarkably contrasting topography: ship and satellite geophysical data reveal steep cliffs and valleys over vast areas, sometimes with elevations of over three kilometres, while other parts of the ocean floor are incredibly flat.

"Seafloor roughness is really important in ocean systems as it influences the circulation and mixing of heat in the water," said Dr Joanne Whittaker, who completed the research as part of her PhD with Professor Dietmar Müller as her supervisor.

When deep ocean water hits peaks and ridges in rough ocean floor, it mixes with higher levels of water and is sometimes forced to rise to the surface, affecting ocean circulation and ultimately the global climate.

"The Earth's surface is made up of a thin rigid skin composed of the tectonic plates, sitting on top of a zone called the mantle where rock is hot enough to flow like warm toffee," explained Dr Whittaker.

"It is known that within this hot mantle, heat circulates on a large scale and affects volcanic eruptions at mid-ocean ridges. Our surprise finding is that supercontinental break-up appears to effect the roughness of the ocean floors."

When the supercontinent Pangaea (made up of all the current continents joined together) broke apart in the Jurassic, while dinosaurs roamed the Earth, new ocean basins were created, eventually forming the Atlantic and Indian Oceans as we know them today. The lavas erupting at these new mid-ocean ridges were unusually hot, resulting in extremely smooth seafloor.

After over 100 million years of molten rock from the mantle erupting and creating smooth seafloors, this giant melt supply system cooled down, making it much harder to create new ocean crust. At this point the ocean floor became more brittle, developing large faults, deepwater hills and underwater mountain ranges.

"We took out the effects of nearby mantle plumes - where molten rock that's hotter than the rest of the mantle rises up within the mantle - and mid-ocean ridge spreading rate, and found that unusually smooth crust formed over mantle that was originally covered by the Pangaea supercontinent. This indicates that a large region of the mantle was relatively hot before break-up of the supercontinent," said Dr Whittaker.

"The speed and direction of tectonic plate separation also effects seafloor roughness. The slower continents move away from each other, the rougher the ocean floor and a shearing motion between tectonic plates also causes large ridges and valleys to form.

"Our work goes a long way towards explaining the physical geography of the ocean basins - a fundamental aspect of the topography of the Earth, considering that oceans cover 70 per cent of Earth's surface!" said Dr Whittaker.

Professor Müller concluded, "Our research provides a framework for understanding the source of seafloor roughness. Understanding why and how the rough patches of ocean floor form is important as they affect deep ocean circulation, with dramatic affects on global climate by influencing ocean mixing."